This project is designed to investigate the structural basis for auditory information processing in the inferior colliculus (IC). The proposed experiments focus on the individual components of the synaptic organization which are: 1) the synaptic contacts formed by afferent axons from the brainstem and neocortex in the IC; 2) the intrinsic circuitry in the IC; and 3) the neurons of the IC whose structure and innervation integrate these inputs to produce responses to auditory signals. In the proposed experiments we will use light (LM) and electron microscopic (EM) techniques to: 1. Identify the synaptic features of brainstem afferent pathways and relate them to binaural interactions in the IC. This includes experiments to: a) Establish the bilateral convergence of cochlear nucleus afferents on single IC neurons with axonal transport and degeneration techniques at the LM and EM level. b) Demonstrate directly the postsynaptic termination of cochlear nucleus afferents on specific cell types in the IC with Golgi-EM and degeneration methods. c) Distinguish the morphology and synaptic contacts made by afferents from the superior olivary complex and dorsal nucleus of the lateral lemniscus. 2. Identify the types of intrinsic axons in the IC through EM analysis of HRP-injected and Golgi-impregnated axons and their endings. 3. Determine which axon types in the IC might use GABA as a transmitter by identifying those which preferentially accumulate 3H-GABA with EM autoradiographic methods. 4. Investigate the dendritic morphology and patterns of synaptic innervation of physiologically characterized, HRP-injected IC neurons with LM and EM techniques. 5. Identify the neocortical afferents to the IC with anterograde axonal transport and EM autoradiography methods.